This invention relates to the field of coating of webs and particularly to the application of a very thin carbon ink coating to a tissue paper web used in the manufacture of one time carbon paper for business forms. Typically, prior art devices for coating carbon paper include a heated coating roller which picks up ink on its surface from a pan of hot ink in which it rotates in a direction opposite to the feed direction of the web. The ink is transferred in a relatively thick layer to the tissue paper web which is guided, while under tension, into contact with the roller. The thick layer is then reduced to a desired thickness by doctor means positioned between the coating roller and a plurality of chill rolls which cause the coating to solidify before it is wound up on a core. Many improvements in doctor devices have been made over the years and such devices are now commonly in the form of a rod, which is usually rotated to distribute wear and is often termed an equalizer rod. Equalizer rods are usually mounted in a groove in a fixed member. Patents which show such devices include U.S. Pat. Nos. 2,001,339, 2,136,739, 2,672,119, 2,695,004, 2,774,329, 3,029,779, 3,063,407, 3,084,663 and 3,304,910. .
Although prior art carbon paper coating machines have been able to produce carbon paper at speeds of as high as about 1,500-1,900 feet per minute, attempts to run significantly faster have been unsuccessful for a number of reasons. One reason for failure has been that high speeds cause flooding and a build up of hydraulic pressure at the equalizer rod which tends to lift the paper and cause too much ink to be left on the paper. The excess ink not only increases cost but is a cause for rejection by the customer since the ink can flake off or smudge or produce fewer legible copies in a typewriter. Smaller diameter rods can produce thinner coatings but are even more prone to flooding. Applying a thinner coating to the paper is not satisfactory since the liquid ink can rip and cause blank areas on the paper.
Another problem has arisen where the carbon paper must be selectively coated to produce uncoated stripes. The uncoated stripes are often required as a surface to which glue may be applied after the paper is slit and is being assembled into business forms. They are also used to provide clean areas for finger gripping or to block out certain information from being reproduced. The stripping is accomplished by thin metal masking strips, preferably brass, which are held at one end so as to lay on top of the rotating coating roller. Small weights mounted above the paper force the paper against the strips and prevent the ink from contacting the paper in the area above the strips. Examples of such strips can be found in U.S. Pat. Nos. 1,476,988, 2,322,533, and 2,330,530. The strips tend to push the ink on the coating roller which is under them to the side so that ink ridges are produced on either side of the uncoated stripes. When the paper is produced in an apparatus where it passes directly from the coating roller to the equalizing rod, ever increasing hydraulic pressures will be produced on the ink coating by the equalizing rod with increases in coating speed. These pressures will force the excess ink in the ridges adjacent the uncoated stripes to move sideways, thus narrowing the stripes and causing the stripe margins to be jagged and irregular.
A third problem caused by the increased hydraulic pressure of high speed contact between the equalizer roll and the ink is that significant quantities of ink particles are propelled upwardly through the pin holes that are usually present in carbonizing tissue. This ink naturally tends to fall back down on the uncoated top side of the carbonized web and results in a dirty product which can transfer ink to the back side of the business forms to which it is assembled and soil the hands of those handling and filing away the forms.